1. Field of the Invention
The present invention relates to a secondary battery and more particularly to a structure of leads welded to the tabs of electrode plates installed within the battery case.
2. Description of the Related Art
FIG. 1 illustrates a prior art lithium ion secondary battery. It is comprised of positive electrode plates, with negative electrode plates, separators between the plates, immersed in an organic electrolyte. The assembled cell 11 is contained in a case 10. To each of the positive electrode plates is welded a tab 13a of aluminum. All of the tabs 13a connected to a positive lead 15a extends out of the case 10 as an external electrical contact. Likewise a plurality of tab 13b of copper are welded to respective negative electrode plates. All of the tabs 13b are connected to a negative lead 15b serving as external electrical contact. Here the leads 15a, 15b have been made of a single material such as aluminum or copper. A protective circuit to prevent overcharging/overdischarging of the battery is further connected to the leads 15a, 15b. The circuit is usually made of nickel and welded to the leads. However, because the leads 15a, 15b are of a single material its welding to the tabs or a protective circuit is poor and the leads have a high resistance.
To solve the above problems, it is an objective of the present invention to provide a secondary battery having an improved connection between battery leads and tabs or a protective circuit as well as low resistance in the electrical conduit in a secondary battery so that, ultimately, energy efficiency of the battery can be increased.
Accordingly, to achieve the above objective, there is provided a secondary battery, which comprising a plurality of positive electrode plates with tabs provided at one ends bundled together, negative electrode plates with tabs provided at one ends bundled together, a separator between each of a positive electrode and a negative electrode, a protective circuit controlling charge/discharge of said battery, and a pair of electrical leads each comprising two layers of different material such that one layer is connected to one of the tab bundles and the other layer is connected to the protective circuit.
It is preferred in the present invention that the layer of one of said pair of electrical leads, connected to the positive tab bundle is of a same material as the positive tabs.
Also, it is preferred in the present invention that said same material is aluminum.
Also, It is preferred in the present invention that the layer of one of said electrical leads connected to the negative tab bundle is of a same material as the negative tabs.
Also, It is preferred in the present invention that said same material is of copper.
Also, It is preferred in the present invention that said two layers are boned to by heat-applied welding.
Also, It is preferred in the present invention that said leads are connected to said protective circuit by ultrasonic welding.
Also, It is preferred in the present invention that said protective circuit and the layer of the leads connected thereto are of nickel.